How Much Does It Cost to Run a Kotatsu?

A kotatsu is a low table with an electric heater fixed under the top and a quilt (kotatsu futon) draped over the frame. Kotatsu heaters are typically rated at 300–600 W.

The rating is the maximum. Once the space under the quilt is warm, the heater cuts its power. Japanese maker Metro Electric lists its 600 W heater at about 180 Wh an hour on high and 70 Wh on low, and its 300 W model at about 160 Wh and 80 Wh.

At 180 Wh an hour, 5 hours a day uses 0.9 kWh: about $0.16 a day in the US and £0.24 in the UK. A month comes to about 27 kWh, or $4.97 in the US. On low it uses well under half that.

Japan isn't in our price table yet. To work out a Japanese bill, multiply these kWh figures by the price per kWh on your electricity bill, or enter it in the calculator. Japan's 100 V supply doesn't change the cost.

A kotatsu heats only the small, enclosed space under the table, not the whole room. The quilt traps the heat around your legs and lower body.

Warming people directly is far cheaper than warming air that keeps leaking out of the room. That's why a kotatsu lets you keep the room itself cooler.

A thick quilt, a rug or futon underneath and a cover on top all hold the heat in, so the heater can run on a lower setting.

The Spanish mesa camilla, a round table with a long skirt and an electric heater (brasero) underneath, works the same way.

Many Japanese homes heat rooms with a wall-mounted air conditioner. Panasonic's model for a 5–6 tatami room (about 8–10 m²) draws about 470 W at its rated heating output, and anything from 125 W to 1,220 W as it speeds up or slows down.

A portable electric heater uses 500–1,500 W while its element is on. On high, a 1,500 W heater uses more in one hour than a kotatsu on high uses in eight.

A kotatsu only warms the people sitting at it. The rest of the room stays cool, so in very cold weather you may still want some room heat for comfort and to limit condensation.

Kotatsu season in Japan runs roughly from November to March.

Hourly figures are the maker's standard averages after warm-up; real use depends on the room temperature, the quilt and the setting. Your heater's rating is on its label. To match these figures in the calculator, enter the rating and about 30% for time running on high.

• Turn it to low or medium once you're warm; on low it uses well under half as much as on high.
• Use a thick kotatsu futon with a cover on top and a rug or futon underneath to hold the heat in.
• Keep the quilt closed around you, and switch the kotatsu off when everyone leaves the table.
• Turn the room heating down a little and let the kotatsu keep you warm.
• Don't use it in place of a bed: it wastes energy and isn't safe.

• Space heater
• Electric blanket
• Heat pump
• Oil-filled radiator
• Bill analyzer